The raw material fluid supply system for the semiconductor manufacturing apparatus and the like needs to supply a process raw material fluid with a stable density to a processing device in terms of improving the quality of a semiconductor product.
Thus, in the former raw material fluid supply system of this type, e.g., in a bubbling type raw material fluid supply system illustrated in FIG. 9, an optical-analysis-type density meter 22 is provided in the vicinity of a raw material vapor outlet of a raw material tank 21 of which temperature is controlled, and the temperature of the raw material tank 21, the flow rate of a carrier gas CG, the vapor pressure Po in the tank, and the like are adjusted by a density detection signal from the density meter 22, whereby a process gas 24 (for example, a process gas containing organometallic material steam, such as trimethyl gallium TMGa, stored in the tank 21) of a predetermined raw material density is supplied to a reactor 23.
FIG. 9 includes thermal mass flow controllers 25 and pressure adjusting devices 26 for the pressure in the tank.
Although density meters 22 having various kinds of configurations are practically used as the optical-analysis-type density meter 22, as illustrated in FIG. 10 (Japanese patent Laid-Open No. 9-178652) and FIG. 11 (Japanese Patent Laid-Open No. 2004-108981), almost the density meters 22 contain an optical cell (gas cell) 27 through which a gas G to be measured flows, a light source 28 which irradiates the inside of the optical cell 27 with light beams, a light receiving device 29 of the light beams passing through the inside of the optical cell 27, an arithmetic calculation device 30 which determines the absorbance from a signal of the light receiving device 29 to calculate the raw material density, and the like. The reference numeral 31 denotes a main pipe line and 32 denotes a branch pipe line.
The density meter 22 of FIG. 10 is configured so that a so-called absorbance of the gas in the optical cell 27 is measured and also the gas density is calculated by applying the Lambert-Beer law to the measurement result of the absorbance.
In the Japanese Patent Laid-Open No. 2004-108981, as illustrated in FIG. 11, an in-line sensor 33 containing an optical cell (light absorption cell) is fixed to the pipeline 31, and the luminous intensity of light transmitting through the optical cell is measured.
The optical-analysis-type density meter 22 and the in-line sensor 33 containing the optical cell (light absorption cell) 27 are known, and thus a detailed description thereof is omitted herein.
Thus, when measuring the raw material gas density, the optical cell 27 first needs to be connected and fixed to the pipeline 32 (or the pipeline 31). However, there are problems that it is not easy to secure the airtightness of a connecting portion between the optical cell 27 and the pipeline 32 (or the pipeline 31) and, for example, it is not easy to achieve connection and fixation having high airtightness by screw connection and a flange connection method employing usual packing materials and seal materials, which makes it actually impossible to achieve securing of airtightness performance (External leak of 1×10−1° Pa·m3/sec or less) demanded in the field of a semiconductor manufacturing apparatus.
In order to continuously perform stable gas density measurement over a long period of time, the transparency of the light transmission window of the optical cell 27 needs to be stable over a long period of time. When the transparency changes with time, the stable gas density measurement becomes difficult to achieve.
However, quartz glass has been used as a constituent material of the light transmission window in former density meters in many cases. Thus, when measuring the density of an organic raw material gas having high corrosiveness or high deposition properties, the light transmission window is corroded or the transparency decreases at an early stage due to the deposition of the raw material. Thus, a problem that stable measurement of the raw material gas density cannot be performed has remained unsolved.
On the other hand, various structures in the optical cell 27 also need to be firmly fixed and held to the main body of the optical cell 27 with high airtightness. Thus, inside the optical cell 27, various kinds of synthetic resin seal materials, silver brazing, gold brazing, and the like are used.
However, materials, such as synthetic resin seal materials, silver brazing, and gold brazing, in the optical cell 27 have risk of serving as emission sources which emits gas and particles contained in each material itself into the organic raw material gas, which actually poses a problem of a reduction in gas purity due to the emission of particles. Thus, in the gas supply systems for semiconductor manufacturing, the use of silver brazing or gold brazing is not desirable.
As described above, the use of the former optical-analysis-type density meter makes it difficult to reduce the size and the cost of facilities and also poses a large number of problems in maintaining gas airtightness, securing of stability of density measurement accuracy, maintenance of high gas purity, and the like. Particularly in the case of an organic raw material gas, the problems, such as the reduction in the transparency of the light transmission window resulting from the corrosiveness thereof, the reduction in the gas purity due to the seal members, the securing of airtightness, and the like, need to be solved as soon as possible.